The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical complications and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and well known to those skilled in the art, have motivated the development of implantable cardiac pacing devices that are wholly contained within a relatively compact package for implant in close proximity to the pacing site, for example, within the right ventricle RV of the heart. With reference to FIG. 1, such a device 100 is illustrated, wherein an hermetically sealed enclosure 105, preferably formed from a biocompatible and biostable metal such as titanium, contains a pulse generator, or an electronic controller and associated power source (not shown), to which at least one electrode 111 is coupled, for example, by a hermetic feedthrough assembly (not shown) like those known to those skilled in the. Enclosure 105 may be overlaid with an insulative layer, for example, medical grade polyurethane, parylene, or silicone, and a portion of the insulation layer may be removed to form another electrode 112, for example, to provide bipolar pacing and sensing in conjunction with electrode 111.
FIG. 1 shows device 100 having been deployed by an operator out from a distal opening 203 of a delivery catheter 200, which the operator has maneuvered up through the inferior vena cava IVC and the right atrium RA into the right ventricle RV. The deployed device 100 is shown fixed at an implant site by a fixation member 115 thereof, but still secured to catheter 200 by a tethering member 280 that extends out from distal opening 203 of catheter 200. Securing device 100 to catheter 200 with tethering member 280 is typically accomplished, prior to maneuvering catheter 200, with device 100 loaded therein, to the illustrated site, by looping tethering member 280 through an attachment feature 121 of device 100 and threading first and second lengths 281, 282 of tethering member 280 through one or more lumens of catheter 200 such that opposing ends thereof protrude out from a proximal opening 201 of catheter 200. After deploying device 100, the operator can grasp the ends of lengths 281, 282, for example, to tug on tethering member 280 to test the fixation of device 100 at the implant site, and/or to apply a greater force to tethering member 280 to remove device 100 from the implant site for repositioning at a more suitable site, if necessary. If satisfied with the implant of device 100, the operator can remove the looped tethering member 280 from engagement with device 100 by releasing, for example, the end of length 281, and then pulling on the end of the other length 282, thereby withdrawing an entirety of length 282 proximally through delivery catheter 200 so that the other length 281 is pulled distally and through device tether attachment feature 121, out from engagement therewith. Such a removal of tethering member 280 may tedious, particularly if blood has clotted around tethering member 280.